.TH gensio 5 01/02/19  "Specifying a gensio"
.SH NAME
gensio \- How to specify a gensio
.SH SYNOPSIS
.B <type>[(options)][,gensio|terminaloptions]
.SH DESCRIPTION
.BR gensio
stands for GENeral Stream Input Output.  It provides an abstraction
for all kinds of stream I/O, and even makes some packet I/O look like
stream I/O (like UDP).  In particular, gensio makes it easy to create
encrypted and authenticated connections.

The
.BR gensio
library specifies a connection (gensio) using a string format.  This
consists of a gensio type, optional options in parenthesis.  For a
terminal gensio (one that is at the bottom of the stack), it may take
more options separated by a comma.  For filter gensios (ones not on
the bottom of the stack) another gensio must be specified after the
comma.  For instance:
.IP
serialdev,/dev/ttyS0
.PP
specifies a serial port gensio.  Or:
.IP
tcp(readbuf=100),localhost,4000
.PP
specifies a TCP connection with a 100 byte read buffer, to connect to
port 4000 on localhost.  Or:
.IP
telnet,tcp,localhost,4000
.PP
specifies a telnet filter on top of a TCP connection.

When specifying a gensio, you can add quotes (single or double) to
remove the special meaning for some characters, so you can have commas
in options and such.  They may also be escaped with a "\\".  For instance,
if you are specifying a laddr in an sctp address, you need to do this.
The following address:
.IP
sctp(laddr=localhost,4001),localhost,3023
.PP
will result in a failure because the option splitting code will split
at the commas.  Instead, do:
.IP
sctp(laddr="localhost,4001"),localhost,3023
.PP
and it will work.

Accepter gensios, gensios used for accepting connections, as opposed
to connecting gensios, are specified in the same way.  Each individual
type can vary, and some gensios are only connecting gensios.  The
options can vary from the accepter and connecting gensios of the same
type.  For instance, an accepting TCP gensio does not have to have a
hostname, but a connecting one does.

When an accepter gensio receives a connection, it will create an
accepted gensio.  This works mostly like a connecting gensio, except
some functions may be limited.  You may not be able to close and then
open an accepted gensio.

The gensio library has a concept of client and server.  The accepted
gensios are generally considered servers.  Connecting gensios are
generally considered clients.  Some gensios may allow this to be
overridden.

A gensio may be reliable or not.  A reliable gensio will reliably
deliver all data in sequence, like TCP.  An gensio that is not
reliable may drop data or deliver data out of sequence, like UDP.
This can be queried with
.B gensio_is_reliable().

A gensio may be packet or not.  A packet gensio will exactly match up
writes and reads.  So if you write 15 bytes into one side, a 15 byte
read for that data will appear on the other side.  A gensio that is
not packet will not respect write boundaries, that 15 byte write may
result in multiple reads or it may be combined with another write into
more than 15 bytes.  Packet I/O requires careful use to make it work
correctly.  Particularly, you must set the max read and write size to
the same value, and you must accept all data on receipt and write
complete packets all at once.

A gensio may be message oriented.  This implementation is stolen from
SCTP (even though it's not really supported on Linux at the moment).
It basically means you can explicitly mark message boundaries when
sending data, and that explicit mark will be set on the read side.
You do this by adding an "eom" auxdata on the write; the end of that
write it assumed to be the end of a message.  If the write does not
accept all the data, the "eom" is ignored, you must write the remaning
data again with "eom" set.  You may also do partial write of messages
and set "eom" at the end.  On the receive side, "eom" will be set when
the end of a message is delivered.  The data delivered in the receive
callback will be only the data for that message.  If the user does not
accept all the data, the data left in the message is again presented
to the user with "eom" set.

The options vary greatly between the different gensios.  Each gensio
type will be covered in a separate section.  Also note that gensio
types can be dynamically added by the user, so there may be gensios
available that are not described here.

Unless othersize noted, every gensio takes a:

.TP
.B readbuf=<n>
option to specify the read buffer size.
.SH "DEFAULTS"
Every option to a gensio (including the serialdev and ipmisol
options), unless othersize stated, is available as a default for the
gensios.  You can use gensio_set_default() to set the default value
used by all gensios allocated after that point.  If you leave the
class NULL, it will set the base default, which will affect all
gensios unless they have an override.  If you set the class, it will
only affect gensios with that class name.

Be very careful with some defaults.  Setting "mode" default, for
instance, could really screw things up.

For string defaults, setting the default value to NULL causes
the gensio to use it's backup default.
.SH "Serial gensios"
Some gensio types support serial port setting options.  Standard
serial ports, IPMI Serial Over LAN, and telnet with RFC2217 enabled.

A client serial gensio can set and get serial port options using the
.B sergensio_xxx()
functions.  Server serial gensios receive requests from the client via
.I GENSIO_EVENT_SER_xxx
events in the callback.
.SH "Streams and Channels"
Some gensios support the concept of a stream and/or a channel.

A stream is delivered as part of the normal data stream of a gensio.
The "default" stream will be treated normally.  All other streams will
have "stream=<x>" given in the auxdata to specify which stream to
write on or which stream was read from.  Streams cannot be
individually flow controlled.

A channel is a flow of data like a stream, but it can be individually
flow controlled.  It appears as a new gensio in the
.I GENSIO\_EVENT\_NEW\_CHANNEL
callback.  You can create a channel with
.B gensio\_alloc\_channel()
and then open it with
.B gensio\_open().
Once open, a channel works like a normal gensio.  If you close the
main channel for a gensio, the other channels will stay open; the
resources for the main channel will still be kept around until all
channels are closed.

See the indvidual gensio description for more information on streams
and channels.
.SH "PUBLIC KEY CRYPTOGRAPHY"
The ssl and certauth gensios use public key cryptography.  This
section gives a little overview of how that works.  You can safely
skip this section if you already understand these concepts.

Public key cryptography is used to authenticate and encrypt
information at the beginning of a session.  It is a fairly expensive
operation and is not generally used to encrypt information after the
beginning.

In public key cryptography, you have three basic things: A private
key, a certificate (or public key), and a certificate authority (CA).

The private key is just that: private.  You don't even send your
private key to a certificate authority for signing of your
certificate.  Keep it private, non-readable by everyone else.  Protect
it, if it becomes known your certificate becomes useless to you, anyone
can impersonate you.

The certificate is the public key, and is mathematically associated
with a single private key.  It's just that, public, anyone can use it
to test that you have the private key by asking you to sign some data.
The data in the certificate (like the Common Name) is part of the
certificate.  If that data is modified, the certificate validation
will fail.

The CA is generally a trusted third-party that validates you and signs
your certificate (generally for a fee).  CAs issue their own public
certificates that are well-known and generally available on your system.
The CA certificates are used to prove that your certificate is valid.
.SS "SIGNING"
The process if signing has been mentioned already, but not described.
Basically, you use your private key to generate a value over some
given data that proves you have the private key.  The certificate is
ised to mathematically verify the signature.

Two things are normally done with this:

In a public key exchange, the entity wishing to be authorized sends a
certificate.  The authorizing entity will look through it's CA for a
certificate that has signed the sent certificate.  If the authorizing
entity finds a certificate that can be used to validate the sent
certificate, the sent certificate is valid.

After that, the authorizing entity sends some generally random data
to the other entity.  The other entity will take that data, perhaps
some other important data that it want to make sure is not modified
in the transfer, and signs that data.  It sends the signature back
to the authorizing entity.  The authorizing entity can then use
the data and the signature to validate that the sending entity has
the private key associated with the certificate.

This is basically how https works.  Note it is the
.B web client
that authenticates the
.B web server,
not the other way around.  This proves that you are connecting to the
entity you say you are connecting to.  The authentication of a web
client to the web server is generally done via a different mechanism
(though SSL/TLS used by the ssl gensio has a way to do it, it is not
generally used for that purpose).

In the web server scenario, data in the certificate (specifically the
Common Name and Subject Alternate Name) must match the name of the web
page to which you are connecting.  The ssl and certauth gensios do not
do this authentication, that is up to the user if it is necessary.
.SS "ENCRYPTING"
The certificate can be used to encrypt data that can only be decrypted
with the private key.  When establishing an encrypted connection, this
is generally used to transfer a symmetric cryptography key from one
entity to another (the authorizing entity to the requesting entity in
the case above).  You could encrypt all the data with the public key,
but that is very expensive and in our example above would require
certificates and private keys on both ends.
.SS "SELF-SIGNED CERTIFICATES"
It is possible to create a certificate that can act as its own
certificate authority.  This is how ssh works.  You create a public
and private key.  You put the public key in the .ssh/authorized_keys
directory on systems where you want to log in.  The certificate acts
as both the public key (as part of the initial transfer) and the CA
(in the authorized_key directory) for authorizing you use of the
system you are logging in to.

ssh also stores places it has connected to in .ssh/known_hosts, which
is the CA in the opposite direction.  This is why it asks you if you
have never connected to a system before, it doesn't have the key in
its CA.  Or why, if you connect to a system you have connected to
before and the certificates don't match or fail validation, it
complains about it.

So if you are using self-signed certificates, you need to be careful
to put only ones you trust in the CA.  This is obviously not possible
in a large system like the world wide web, thus the creation of
third-party trusted CAs.
.SS "TRUST AND CRYPTOGRAPHY"
The above discussions mention trust several times.  Cryptography does
not remove the need for trust.  It just makes trust more convenient.
If someone sends you a certificate, you need to validate that it was
actually them that sent you the certificate, and that it was not
modified in transit.  If you add that certificate to your CA, you are
trusting the certificate, and you better make sure (with fingerprints,
generally, see the openssl docs for details) that it came from a
trusted entity.

The hardest part of cryptography in general is key management.
Breaking cryptographic algorithms is really hard.  Getting people to
divulge private keys or use the wrong keys is a lot easier.

For more on cryptography in general, and cryptography and trust, Bruce
Schneier has some excellent books on the subject.
.SH "IPv6, IPV4, and host names"
Note that a single hostname may result in more than one address.  For
instance, it may have both an IPv4 and IPv6 address.  These are
treated just like multiple hostnames.  The hostname may be prefixed
with ipv4 or ipv6, which will force the connections to those
protocols.  Specifying ipv6n4 will create a socket that is IPv6 but
will handle IPv4 connections.  This is the default.

In general IPv6 addresses are preferred if both are available.

If you do not specify a hostname on an accepting gensio (like
sctp,1234) it will only create an IPv6 socket that is IPv4 mapped.
Even though getaddrinfo would normally return two addresses, only the
IPv6 one is used unless there are no IPv6 addresses configured where it
will return an IPv4 address.

In general, for connecting gensios only the first address that is
found will be used.  SCTP is the exception, it will do multi-homing on
all the addresses that come up.  Do you may need to be fairly specific
with addresses.
.SH "TCP"
.B tcp[(<options>)][,<hostname>],<port>[[,<hostname>],<port>[...]]
.br
.B hostname = [ipv4|ipv6|ipv6n4,]<name>

A TCP connecting gensio must have the hostname specified.  Mulitiple
hostname/port pairs may be specified.  For a connecting TCP gensio,
each one will be tried in sequence until a connection is established.
For acceptor gensios, every specified hostname/port pair will be
listened to.
.SS Dynamic Ports
For accepters, if the port is specified as zero, a random port in the
dynamic port range specified by IANA will be chosen.  If more than one
address is present, the same port will be chosen for all addresses.
You can fetch the port using the gensio_acc_control() function with
the option GENSIO_ACC_CONTROL_LPORT.

.SS Out Of Band Data
TCP supports out of band (oob) data, which is data that will be
delivered out of order as soon as possible.  This comes in a normal
read, but with "oob" in the auxdata.  You can send oob data by adding
"oob" to the write auxdata, but oob writes are limited to 1 byte and
writing any more than this results in undefined behavior.  Note that
"oobtcp" is also delivered and accepted in auxdata, so you can tell
TCP oob data from other oob data.
.SS Options
In addition to readbuf, the tcp gensio takes the following options:
.TP
.B nodelay[=true|false]
Sets nodelay on the socket.
.TP
.B laddr=<addr>
An address specification to bind to on the local socket to set the
local address.
.TP
.B reuseaddr[=true|false]
Set SO_REUSEADDR on the socket, good for accepting gensios only.
Defaults to true.
.TP
.B tcpd=on|print|off
Accepter only, sets tcpd handling on the socket.  If "on", tcpd is
enforced and the connection is just closed on a tcpd denial.  "print"
is like on, except it writes "Access Denied" on the socket before
closing it.  "off" disabled tcpd handling on the socket.  Defaults to
on.  Not available if tcpd is disabled at compile time.
.TP
.B tcpdname=<name>
Accepter only, sets the name to use for tcpd access control.  This
defaults to "gensio", and the default can be overriden with
gensio_set_progname().  This option allows you to override it on a
per-gensio accepter basis.  Not available if tcpd is disabled at
compile time.
.SS Remote Address String
The remote address will be in the format "[ipv4|ipv6],<addr>,<port>" where the
address is in numeric format, IPv4, or IPv6.
.SS Remote Address
TCP returns a standard struct sockaddr for GENSIO_CONTROL_RADDR_BIN
control.
.SH "UDP"
.B udp[(<options>)][,<hostname>],<port>[[,<hostname>],<port>[...]]
.br
.B hostname = [ipv4|ipv6|ipv6n4,]<name>

A UDP gensio creates a UDP socket, but it makes it look like an
unrealiable stream of data.  The specification is the same as a TCP
socket, except that a UDP socket is created, obviously.

The semantics of a UDP socket are a little bit strange.  A connecting
UDP socket is fairly straightforward, it opens a local socket and
sends data to the remote socket.

An accepter gensio is not so straightforward.  The accepter gensio
will create a new accepted gensio for any packet it receives from a
new remote host.  If you disable read on any of the accepted gensio or
disable accepts on the accepting gensio, it will stop all reads on all
gensios associated with that accepting gensio.

Note that UDP accepter gensios are not really required for using UDP,
the are primarily there for handling ser2net accepter semantics.  You
can create two connecting UDP gensios and communicate between them.

UDP gensios are not reliable, but are obviously packet-oriented.

Port 0 is supported just like TCP for accepters, see Dynamic Ports in
the TCP section above for details.

The destination address defaults to the one specified on the gensio
specifier (for connecting gensios) or the remote address that
initiated the connection (for accepting gensios), but may be
overridden using "addr:<addr>" in the write auxdata.
.SS Options
In addition to readbuf, the udp gensio takes the following options:
.TP
.B laddr=<addr>
An address specification to bind to on the local socket to set the
local address.
.TP
.B nocon[=true|false]
Don't be connection oriented, just receive all packets and deliver
them without establishing connections.  Only valid for the client
gensio.  The receive address is passed into the auxdata prefixed by
"addr:", this is the address formatted by gensio_addr_to_str().
.TP
.B mloop[=true|false]
If false, multicast packets transmitted will not be received on the
local host.  If true, they will.
.TP
.B mcast=<addr>
Add an address to receive multicast packets on.  There is no port
number, this is just addresses.  You can specify multiple addresses in
a single multicast option and/or the multicast option can be used
multiple times to add multiple multicast addresses.
.TP
.B reuseaddr[=true|false]
Set SO_REUSEADDR on the socket, good for connecting and accepting
gensios.  Defaults to false.
.SS "Remote Address String"
The remote address will be in the format "[ipv4|ipv6],<addr>,<port>" where the
address is in numeric format, IPv4, or IPv6.
.SS "Remote Address"
UDP returns a standard struct sockaddr for GENSIO_CONTROL_RADDR_BIN
control.
.SS "UDP Multicast"
Multicast can be used on UDP gensios with the
.B nocon, maddr
and
.B laddr
options.  To set up a multicast, create a client UDP gensio and set
the laddr for the receive port and the destination address to the
multicast and enable nocon, like:
.IP
"udp(mcast='ff02::1',laddr='ipv6,3000',nocon),ff02::1,3000"
.PP
and you will receive and send data on the multicast address.  The
.B laddr
option is required to set the port to receive on.  It means you will
have a local address, too, and will receive packets on that, too.
.SH "SCTP"
.B sctp[(<options>)][,<hostname>],<port>[[,<hostname>],<port>[...]]
.br
.B hostname = [ipv4|ipv6|ipv6n4,]<name>

An SCTP gensio is specified like a UDP or TCP one.  However, the
semantics are different.  For a connecting gensio, it will attempt to
create a multi-homed connect with all the specified hostnames and
ports.  All the ports must be the same.

For an accepter gensio, it will create a single socket with all the
specified addresses as possible destinations.  Again, all the ports
must be the same.

SCTP gensios are reliable.  They are not, at the moment, packet
oriented.  There is currently no support of SCTP_EXPLICIT_EOR in the
Linux implementation of SCTP, and without that it would be hard to
make it packet oriented.

When specifying IPv6 addresses that might map to IPv4, you must be
careful.  If one side can do IPv4 and the other side can only do IPv6,
the connection may come up, but will disconnect quickly because it
cannot communicate on the IPv4 side.  For instance, the following
accepter:
.IP
tools/gensiot -a "sctp,ipv6,::,1234"
.PP
and the following connector:
.IP
tools/gensiot "sctp,::1,1234"
.PP
will fail this way because the connector will support IPv4 add but the
accepter will not.
.SS Nagle and SCTP
SCTP implements the Nagle algorithm by default, which can interact
badly if
.B sack_freq
is set to more than one.  At least Linux defaults
.B sack_freq
to 2, but the gensio overrides this to avoid surprising behaviour.
What happens is in some situations you can get an outstanding packet
that is unacked, since
.B sack_freq
is greater than one.  The Nagle algorithm will not send any new data
until any already sent data is acked.  So one end is waiting for a new
packet to send a sack, and the other end is holding data until it gets
a sack.  So you get stuck waiting for the
.B sack_delay
where the sack will go out and kick things back off again.

You need to be aware of this if you modify sack_freq.
.SS Options
In addition to readbuf, the sctp gensio takes the following options:
.TP
.B instreams=<n>
.TP
.B ostreams=<n>
These specify the number of incoming and outgoing streams for the
connection.  The default is one.  The stream is given in the auxdata
for read and write in the format "stream=<n>".
.TP
.B sack_freq=<n>
.TP
.B sack_delay=<n>
These specify the handling of selective acknowledgements (sacks).
.B sack_freq
sets the number of outstanding packets that must be received before
sending a sack.  The default is 1, meaning it doesn't wait at all.
.B sack_delay
sets the maximum time before a sack is sent if outstanding packets are
present, in milliseconds.  The default is 10, but this is disabled if
.B sack_freq
is set to 1.  Setting either of these to 0 enables the system
defaults.
.TP
.B nodelay[=true|false]
Sets nodelay on the socket.
.TP
.B laddr=<addr>
An address specification to bind to on the local socket to set the
local address.
.TP
.B reuseaddr[=true|false]
Set SO_REUSEADDR on the socket, good for accepting gensios only.
Defaults to true.

Port 0 is supported just like TCP for accepters, see Dynamic Ports in
the TCP section above for details.

SCTP support out of band (oob) data, which is data that will be
delivered out of order as soon as possible.  This comes in a normal
read, but with "oob" in the auxdata.  You can send oob data by adding
"oob" to the write auxdata.

See documentation on SCTP for more details.
.SS "Remote Address String"
The remote address will be in the format
"[ipv4|ipv6],<addr>,<port>[;[ipv4|ipv6],<addr>,<port>[...]]"
where the address is in numeric
format, IPv4, or IPv6.  Each remote address for the SCTP connection is
listed.
.SS "Remote Address"
SCTP returns a packed struct sockaddr for GENSIO_CONTROL_RADDR_BIN
control, per SCTP semantics.
.SH "UNIX"
.B unix[(<options>)],<socket_path>

Create a unix domain socket as an accepter, or connect to a unix
domain socket as a connecter.

A file will be created with the given socket path, you must have
permissions to create a writeable file in that location.  If the file
already exists, an error will be returned on an accepter socket unless
you specify
.I delsock
which will cause the file to be deleted.

You should read the unix(7) man page for details on the semantics of
these sockets, especially permissions.  The options below allow
setting various permission and ownership of the file, but this may not
have any effect on who can open socket depending on the particular
operating system.
.B Portable programs should not rely on these permissions for security.
Also note that Linux remote credentials are not
currently implemented.
.SS Options
In addition to readbuf, the unix gensio takes the following options:
.TP
.B delsock[=true|false]
If the socket path already exists, delete it before opening the socket.
.TP
.B umode=[0-7|[rwx]*]
Set the user file mode for the unix socket file.  This is the usual
read(4)/write(2)/execute(2) bitmask per chmod, but only for the user
portion.  If a mode is specified, all other modes default to "6" (rw)
+unless they are specified, and the final mode is modified by the umask
+per standard *nix semantics.  If no mode is specified, it is set to
+the default and not modified.  Note that the perm option below is
+probably a better way to set this.
.TP
.B gmode=[0-7|[rwx]*]
Set the group file mode for the unix socket file, see umode for details.
.TP
.B omode=[0-7|[rwx]*]
Set the other file mode for the uix socket file, see umode for details.
.TP
.B perm=[0-7][0-7][0-7]
Set the full mode for the unix socket file per standard *nix
semantics, modified by umask as the above mode operations are.
.TP
.B owner=<name>
Set the owner of the unix socket file to the given user.
.TP
.B group=<name>
Set the group of the unix socket file to the given group.
.SS Remote Address String
The remote address will be: "unix,<socket path>".
.SS Remote Address
UNIX returns a standard struct sockaddr_un for GENSIO_CONTROL_RADDR_BIN
control.
.SH "serialdev"
.B serialdev[(<options>)],<device>[,<serialoption>[,<serialoption>]]

A serialdev connection is a local serial port.  The device is a
.B /dev/xxx
type, and should be real stream device of some type that normal
termios work on (except for wronly).

This is, no surprise, a serial gensio.

One problem with serialdev and UUCP locking is that if you fork() a
process while one is open, the forked process will have the serialdev
but the value in the UUCP lockfile will be incorrect.  There's not
much that can be done about this, so be careful.
.SS Options
In addition to readbuf, the serialdev gensio takes the following options:
.TP
.B nouucplock[=true|false]
disables UUCP locking on the device.  Useful for /dev/tty, which shouldn't
use locking.  This is not available as a default.
.PP
There are a plethora of serialoptions, available as defaults:
.TP
.B [speed=]<speed><parity><databits><stopbits>
This is a normal serial port configuration specification, like
"9600N81".  The "speed=" at the beginning is optional, but "speed" is
a default type for this.
.TP
.B wronly[=true|false]
Set the device to write only.  No termios definition is done on the
port.  This can be done to talk to a line printer port, for instance.
False by default.
.TP
.B nobreak[=true|false]
Clear the break line at start (or don't clear it).  Default it to not
clear it (false).
.TP
.B rs485=off|<delay rts before send>:<delay rts after send>[:<conf>[:<conf>]]
Set up RS-485 for the serial port.  The first two parameters set the
RTS delay (in milliseconds) of RTS before and after sending.  The conf
values can be: "rts_on_send" - RTS set when sending, "rts_after_send" -
RTS set after sending, "rx_during_tx" - can receive and transmit at
the same time, and "terminate_bus" - enable bus termination.

Using "off" will disable rs485, that is useful for overriding a default.
default is "off".
.TP
.B xonxoff[=true|false]
Enable/disable xon/xoff flow control.  Default is false.
.TP
.B rtscts[=true|false]
Enable/disable rts/cts flow control.  Default is false.
.TP
.B local[=true|false]
Ignore/don't ignore the modem control lines.  The default it to not
ignore them (false).  However, if you don't ignore the modem control
lines, it can result in long shutdown delays.
.TP
.B hangup-when-done[=true|false]
Lower/don't lower the modem control lines when the gensio is closed.
The default is to not lower the modem control lines on close (false).
.B custspeed[=true|false]
Allow/don't allow setting of custom baud rates.  Ignored if custom
baud rates are not supported.  Normally only standard baud rates are
supported (1200, 2400, etc).  If supported by the hardware, this
allows any arbitrary value to be set.
.SS "Remote Address String"
The remote address string is the device and serial parms, in a format
like "9600N81[,<option>[,<option>[...]]]".  Options are one of:
XONXOFF, RTSCTS, CLOCAL, HANGUP_WHEN_DONE, RTSHI, RTSLO, DTRHI, DRLO,
offline.
.SS "Remote ID"
The remote ID for the serial dev is the file descriptor returned as an
integer.
.SH "stdio"
accepter =
.B stdio[(options)]
.br
connecting =
.B stdio[(options)],<program>

The stdio gensio is a fairly strange one, but it's fairly useful.

A connecting stdio gensio runs the given program (unless self is set
as an option) and connects its standard input and output to the
gensio's main channel.  So it's easy to run a program and interact
with it.  If you want to get stderr from the gensio, open a channel on
the main channel gensio, see below.

.B NOTE:
Though epoll() doesn't work with normal files, the stdio gensio has
special handling for normal files so they mostly work.  This can have
surprising side effects if you use stdin as a normal file.  When you
hit the end of stdin, the stdio gensio will return GE_REMCLOSE and you
may shut down the gensio and possibly lose any output going to
stdout.  Use the file gensio if you can.

For connecting gensios, in the forked process, the code will set the
uid and guid to the current set effective uid and guid if the
effective and real uids are different.  This way a user can set the
effective uid and gid to what they want the program to run under, but
keep the uid and gid to the (probably root) values so they can be
restored to those values after opening the pty.  The group list is
also set to the groups the effective userid is in.  Note that nothing
is done if the effective and real userids are the same.

If you have both stdin/stdout and stderr opened, you must close both
of them to completely close the gensio.  You cannot re-open the gensio
until both are closed.

The connecting gensio support the GENSIO_CONTROL_ENVIRONMENT control
to allow the environment to be set for the new process.

An accepting gensio immediately does a connection when started and
connection stdin and stdout of the running program to the gensio.
.SS Options
In addition to readbuf, a connecting stdio takes the following options:
.TP
.B self[=true|false]
Instead of starting a program, connect to the running program's stdio.
This is the same as an accepting stdio gensio, except you don't have
to go through the accept process.
.TP
.B raw[=true|false]
For a "self" or accepter version of the gensio, put the I/O in "raw"
mode, meaning character at a time, turn off ^C, etc.  This will fail
on pipes or files.
.TP
.B stderr-to-stdout
Send standard error output to standard out instead of having a separate
channel for it.
.TP
.B noredir-stderr
Do not modify the stderr for the program, use the calling program's
stderr.  This can be useful if you want to see stderr output from a
program.
.SS "Channels"
The stdio connecting gensio that start another program does not
provide stderr as part of the main gensio. You must create a channel
with
.B gensio_alloc_channnel()
and then open it get stderr output.  The args for
.B gensio_alloc_channel()
may be an optional "readbuf=<num>" and sets the size of the input
buffer.
.SS "Remote Address String"
The remote address string is either "stdio,<args>" for the main channel or
"stderr,<args>" for the error channel.  The args will be a set of quoted
strings with a space between each string, one for each argument,
with '"' around each argument, each '"' in the string converted
to '\\"' and each '\\' in the string converted to '\\\\'.
.SS "Remote ID"
The remote ID is the pid for the remote device, only for connecting
gensios that start a program, not for any other type.
.SH "echo"
connecting =
.B echo[(options)]

The echo gensio just echos back all the data written to it.  Useful
for testing.
.SS Options
In addition to readbuf, a connecting echo takes the following options:
.TP
.B noecho[=true|false]
Instead of echoing the data, just become a data sink.
.SS "Remote Address String"
The remote address string is "echo".
.SH "file"
connecting =
.B file[(options)]

The file gensio opens an (optional) input file and (optional) output
file for the gensio.  If you need to read/write a file in gensio, you
.B must
use this.  Semantics on files for stdio do not alway work correctly.

If an input file is specified, data will be read from the input file
and delivered on the read interface of the gensio until end of file,
when the GE_REMCLOSE error will be returned.

If an output file is specified, data will be written to the output
file on writes.  The output file is always ready to receive data.
.SS Options
In addition to readbuf, a connecting file takes the following options:
.TP
.B infile=filename
Set the input filename.
.TP
.B outfile=filename
Set the output filename.
.TP
.B read_close[=true|false]
If this is true (the default), cause a GE_REMCLOSE error on read when
all the file data is ready.  If false, just stop returning data and
don't do the GE_REMCLOSE.  This is useful if you just want to inject a
bunch of data then do nothing.
.TP
.B create[=true|false]
Create the output file if it does not exist.
.TP
.B umode=[0-7|[rwx]*]
Set the user file mode for the file if the file is created.  This is
the usual read(4)/write(2)/execute(2) bitmask per chmod, but only for
the user portion.  If a mode is specified, all other modes default to "6" (rw)
+unless they are specified, and the final mode is modified by the umask
+per standard *nix semantics.  If no mode is specified, it is set to
+the default and not modified.  Note that the perm option below is
+probably a better way to set this.
.TP
.B gmode=[0-7|[rwx]*]
Set the group file mode for the file if the file is created, see umode
for details.
.TP
.B omode=[0-7|[rwx]*]
Set the other file mode for the file if the file is created, see umode
for details.
.TP
.B perm=[0-7][0-7][0-7]
Set the full mode for the file per standard *nix semantics, modified
by umask as the above mode operations are.
.SS "Remote Address String"
The remote address string is "file([infile=<filename][,][outfile=<filename>])".
.SH "dummy"
accepter =
.B dummy

The dummy gensio accepter is useful for place holding, where you don't
want a real working accepter but need to put something.

As you might guess, it doesn't do anything.
.SH "ipmisol"
.B ipmisol[(options)],<openipmi arguments>[,ipmisol option[,...]]

An ipmisol gensio creates an IPMI Serial Over LAN connection to an
IPMI server.  See the OpenIPMI documentation for information on the
arguments.

This is a serial gensio, but the baud rate settings are fairly limited.
.SS Options
In addition to readbuf, the ipmisol gensio takes the following options:
.TP
.B writebuf=<n>
to set the size of the write buffer.
.PP
It also takes the following ipmisol options:
.TP
.B 9600, 19200, 38400, 57600, 115200
Baud rate, 9600 by default.  Anything after the number is ignored, for
compatibility with things like "N81".  You cannot set those values for
ipmisol, they are fixed at "N81".
.TP
.B nobreak[=true|false]
Disable break processing.  False by default.
.TP
.B authenticated[=true|false]
Enable or disable authentication.  True by default.
.TP
.B encrypted[=true|false]
Enable or disable encrypted.  True by default.
.TP
.B deassert-CTS-DCD-DSR-on-connect[=true|false]
False by default
.TP
.B shared-serial-alert=fail|deferred|succeed
Behavior of handling serial alerts.  fail by default.
.TP
.B ack-timeout=<number>
The time (in microseconds) when an ack times out.  1000000 by default.
.TP
.B ack-retries=<number>
The number of times (ack-timeouts) an ack is re-sent before giving up.
10 by default.
.SH "telnet"
accepter =
.B telnet[(options)]
.br
connecting =
.B telnet[(options)]

A telnet gensio is a filter that sits on top of another gensio.  It
runs the standard telnet protocol andn support RFC2217.
.SS Options
In addition to readbuf, the telnet gensio takes the following options:
.TP
.B writebuf=<n>
set the size of the write buffer.
.TP
.B rfc2217[=true|false]
enable or disable RFC2217 mode for this gensio.  If this is enabled
and the remote end of the connection supports RFC2217 also, the gensio
will be set up as a serial gensio and you can do normal serial gensio
handling on it.
.TP
.B mode=client|server
Set the telnet mode to client or server.  This lets you run a telnet
server on a connecting gensio, or a telnet client on an accepter
gensio.
.SS "Remote info"
telnet passes remote id, remote address, and remote string to the child
gensio.
.SH "ssl"
accepter =
.B ssl[(options)]
.br
connecting =
.B ssl[(options)]

An SSL gensio runs the SSL/TLS protocol on top of another gensio.
That gensio must be reliable.

Use is pretty straightforward.  The hardest part about using the SSL
gensio is the certificates.  A server SSL gensio must be given a
certificate and a key.  A client SSL gensio must be given a
certificate authority.  A client will user the certificate authority
to verify that the server has the proper certificate and keys.

The gensio has options to have the server request the certificate from
the client and validate it.

SSL gensios are reliable.  They are also packet-oriented.
.SS Options
In addition to readbuf, the SSL gensio takes the following options:
.TP
.B writebuf=<n>
set the size of the write buffer.
.TP
.B CA=<filepath>
Set a place to look for certificates for authorization.  If this ends
in a "/", then this is expected to be a directory that contains files
with certificates that must be hashed per OpenSSL (see the "openssl
rehash" command for details.  Otherwise it is a single file that
contains one or more certificates.  The default CA path is used
if not specified.  Note that setting this to an empty string disables
it, so you can override a default value if necessary.
.TP
.B key=<filename>
Specify the file to get the private key for the server.  This is
required for servers.  It may be specified for clients, and is required
for the client if the server requires a certificate (it has CA set).
.TP
.B cert=<filename>
Specify the file that contains the certificate used for the
connection.  If this is not specified, the certificate is expected to
be in the key file.  Note that setting this to an empty string disables
it, so you can override a default value if necessary.
.TP
.B mode=client|server
Normally an accepter gensio is in server mode and a connecting gensio
is in client mode.  This can be used to switch the roles and run in
SSL server mode on a connecting gensio, or vice versa.
.TP
.B clientauth[=true|false]
Normally a client is not authorized by the server.  This requires
that the client provide a certificate and authorizes that certificate.
Ignored for client mode.
.TP
.B allow-authfail[=true|false]
Normally if the remote end certificate is not valid, the SSL gensio
will close the connection.  This open allows the open to succeed with
an invalid or missing certificate.  Note that the user should verify
that authentication is set using gensio_is_authenticated().

Verification of the common name is
.B not
done.  The application should do this, it can fetch the common name
and other certificate data through a control interface.

You can use self-signed certificates in this interface.  Just be aware
of the security ramifications.  This gensio is fairly flexible, but
you must use it carefully to have a secure interface.

The SSL gensio will call the gensio event callback (client) or the
gensio acceptor event callback (server) after the certificate is
received but before it is validated with the
GENSIO_EVENT_PRECERT_VERIFY or GENSIO_ACC_EVENT_PRECERT_VERIFY events.
This allows the user to validate data from the certificate (like
common name) with GENSIO_CONTROL_GET_PEER_CERT_NAME or set a
certificate authority for the validation with GENSIO_CONTROL_CERT_AUTH.
.SS "Remote info"
ssl passes remote id, remote address, and remote string to the child
gensio.
.SH "certauth"
accepter =
.B certauth[(options)]
.br
connecting =
.B certauth[(options)]

A certauth gensio runs an authentication protocol to on top of
another gensio.  That gensio must be reliable and encrypted.

This is like the reverse of SSL.  The client has a key and certificate,
the server has a certificate authority (CA).  This also supports password
authentication.

Once authentication occurs, this gensio acts as a passthrough.  The
readbuf option is not available in the gensio.
.SS Options
The certauth gensio takes the following options:
.TP
.B CA=<filepath>
Set a place to look for certificates for authorization.  If this ends
in a "/", then this is expected to be a directory that contains files
with certificates that must be hashed per OpenSSL, see the gensio-keygen(1)
in the rehash section for details.  Otherwise it is a single file that
contains one or more certificates.  The default CA path for openssl is used
if not specified.  This is used on the server only, it is ignored
on clients.  Note that setting this to an empty string disables
it, so you can override a default value if necessary.
.TP
.B key=<filename>
Specify the file to get the private key for the client.  This is
required for clients.  It is ignored on server.
.TP
.B cert=<filename>
Specify the file to get the private key for the client.  This is
required for clients.  It is ignored on server.  If this is not
specified, the certificate is expected to be in the key file.  Note
that setting this to an empty string disables it, so you can override
a default value if necessary.
.TP
.B username=<name>
Specify a username to authenticate with on the remote end.  This is
required for the client.  It is ignored on the server.
.TP
.B service=<string>
Set the remote service requested by the client.  Optional, but the
other end may reject the connection if it is not supplied. Ignored on
the server.
.TP
.B password=<string>
Specify the password to use for password authentication.  This is not
recommended, the callback is more secure to use.
.TP
.B mode=client|server
Normally an accepter gensio is in server mode and a connecting gensio
is in client mode.  This can be used to switch the roles and run in
server mode on a connecting gensio, or vice versa.
.TP
.B allow-authfail[=true|false]
Normally if the remote end certificate is not valid, the certauth gensio
will close the connection.  This open allows the open to succeed with
an invalid or missing certificate.  Note that the user should verify
that authentication is set using gensio_is_authenticated().
.TP
.B use-child-auth[=true|false]
If the child gensio is authenticated, then do not run the protocol, just
go straight into passthrough mode and don't do any authentication.
.TP
.B enable-password[=true|false]
On the server, allow passwords for login.  On the client, send a
password if asked for one.  By default passwords are disabled.
Use of passwords is much less secure than certificates, so this
is discouraged.
.PP
Verification of the common name is
.B not
done.  The application should do this, it can fetch the common name
and other certificate data through a control interface.  It may also
use the username fetched through the control interface.

You can use self-signed certificates in this interface.  Just be aware
of the security ramifications.  This gensio is fairly flexible, but
you must use it carefully to have secure authentication.

The certauth gensio has 4 major callbacks dealing with authentication
of the user.  They may or may not be called depending on the
circumstances.  The normal events come in if you have allocated a
gensio and are doing an open.  The _ACC_ events come in if it is
coming in from an accept and there is no gensio reported yet.  In the
_ACC_ case, be careful, do not use the given gensio for anything but
checking certificate and username parameters, and do not save it.

All these calls should return 0 if they want the authentication to
immediately succeed, EKEYREJECTED if they reject the authentication,
ENOTSUP if they want certauth to ignore that part of the authentication,
or any other errno will result in the connetion being shut down.

The callbacks are:
.TP
.B GENSIO_EVENT_AUTH_BEGIN / GENSIO_ACC_EVENT_AUTH_BEGIN
On the server side, this is called when to authentication is requested
buy the client.  The username will be available if the user provided
it via GENSIO_CONTROL_USERNAME.
.TP
.B GENSIO_EVENT_PRECERT_VERIFY / GENSIO_ACC_EVENT_PRECERT_VERIFY
On the server side, thi is called after the certificate has been
received but before it is verified.  The user can use this to query
the certificate and update the certificate authority based on username
or certificate information.
.TP
.B GENSIO_EVENT_VERIFY_PASSWORD / GENSIO_ACC_EVENT_VERIFY_PASSWORD
On the server side, this is called if the certificate verification has
failed and after the password has been requested from the remote end.
The password is passed in, it is cleared immediately after this call.
.TP
.B GENSIO_EVENT_REQUEST_PASSWORD / GENSIO_ACC_EVENT_REQUEST_PASSWORD
On the client side, this is called if the server requests that a
password be sent and the password is not already set for the gensio.
The requested password is immediately cleared after being sent to
the server.
.SS "Remote info"
certauth passes remote id, remote address, and remote string to the child
gensio.
.SH "mux"
accepter =
.B mux[(options)]
.br
connecting =
.B mux[(options)]

A mux gensio is a gensio filter that allows one or more channels to be
created on top of a single gensio, multiplexing the connection.  Each
channel has full individual flow-control.  The channels are message
oriented as described above, and use can use a mux without additional
channels to just do message demarcation.  They also support out-of-bounds
messages.
.SS Options
A mux gensio takes the following options:
.TP
.B max_channels=<n>
Allow at most <n> channels to be created in the mux.  The default is 1000.
The minimum value of <n> is 1, the maximum is 65536.
.TP
.B service=<string>
Set the remote service requested by the client.  Optional, but the
other end may reject the connection if it is not supplied. Ignored on
the server.
.TP
.B mode=client|server
By default a mux accepter is a server and a mux connecter is a client.
The protocol is mostly symmetric, but it's hard to kick things off
properly if both sides try to start things.  This option lets you
override the default mode in case you have some special need to do so.
.PP
When the open is complete on the mux gensio, it will work just like a
transparent filter with message demarcation.  In effect, you have
one channel open.
.SS "Creating Channels"
To create a channel, call the
.B gensio_alloc_channel()
function on the mux gensio.  This will return a new gensio that is a
channel on the mux gensio.  You can pass in arguments, which is an
array of strings, currently
.B readbuf, writebuf,
and
.B service
are accepted.  The service you set here will be set on the remote channel
so the other end can fetch it.  The new channel needs to be opened with
.B gensio_open()
before it can be used.

As you might imaging, the other end of a mux needs to know about the
new channel.  If one end (either end, doesn't matter) calls
.B gensio_alloc_channel()
and then opens the new channel, the other end will get a
.I GENSIO\_EVENT\_NEW\_CHANNEL
event as described in the
.B Streams and Channels
section.  You can call it using any mux channel.  The first element in
the auxdata is the service.

You can modify the service value after you allocate the channel but
before you open it.
.SS "Out Of Band Messages"
mux support out of band (oob) data, which is data that will be
delivered normally.  This comes in a normal read, but with "oob" in
the auxdata.  You can send oob data by adding "oob" to the write
auxdata.  You should normally use the "eom" flag so the end of the out
of band messages ismarked.

This is so you can send special data outside of the normal processing
of data.
.SS "Mux Events"
As you might imaging, normal data events come through the gensio
channel they are associated with.  Close events will, too.  If a mux
gensio closes abnormally (like the underlying connection fails) you
will get a read error on each channel.

New channel events (and other global events coming from lower gensios,
like if you are running over ssl or telnet) come in through the
original gensio normally.  However, you can close that, another
channel will be chosen to receive those event.  In general, it's best
to handle the global events on all channels and not assume.
.SS "Closing and Freeing a Mux"
To close a mux gensio, just close all channels associated with it.
There is no global close mechanism (you would not believe the complexity
that adds).  Once you have closed a mux gensio, you can re-open it with
.B gensio_open().
It will not recreate all the channels for you, you will have one channel,
and the channel you use to call
.B gensio_open()
will become the first channel.

You cannot re-open individual channels.

To free a mux gensio, you must free all the channels on it.
.SH "pty"
connecting =
.B pty[(options)][,<program>]

Create a pty and optionally run a program on the other end of it.
With a program specified, this is sort of like stdio, but the program
is running on a terminal.  Only connection gensios are supported.

pty has some unusual handling to allow execution of login shells of
users from root.

If the first character of the program is '-', it is removed from the
execution of the command but left in argv[0].  This will cause a shell
to act as a login shell.

In the forked process, the pty code will set the uid and guid to the
current set effective uid and guid if the effective and real uids are
different.  This way a user can set the effective uid and gid to what
the want to pty to run under, but keep the real uid and gid to the
(probably root) values so they can be restored to those values after
opening the pty.  The group list is also set to the groups the
effective userid is in.  Note that nothing is done if the effective
and real userids are the same.

The pty gensio supports the GENSIO_CONTROL_ENVIRONMENT control to allow
the environment to be set for the new process.

If no program is specified, when the pty gensio is opened it will just
create the pty but won't attach anything to it.  Another program can
open the pty.  You can get the slave pty device by getting the local
address string.
.SS Options
In addition to readbuf, the pty gensio takes the following options.
These options are only allowed if the pty is unattached.  ptys with
programs run on them need to follow the standard semantics.
.TP
.B link=<filename>
create a symbolic link from filename to the pty name.  This way you
can have a fixed reference to refer to the pty.  Standard permissions
apply.  Without forcelink the open will file if filename already
exists.
.TP
.B forcelink[=true|false]
if link is specified, if a file already exists where the symbolic link
is, replace it.  Be careful, it deletes whatever is there
indiscriminately.
.TP
.B raw[=true|false]
causes the pty to be set in raw mode at startup.  This is important for
anything that will start writing immediately to the pty.  If you don't
set this, echo will be on and writes on the master will be echoed back.
In general, this is useful for unattached ptys.  It was added for testing,
but will be usedful in many situations.
.TP
.B umode=[0-7|[rwx]*]
Set the user file mode for the pty slave.  This is the usual
read(4)/write(2)/execute(2) bitmask per chmod, but only for the user
portion.  If a mode is specified, all other modes default to "6" (rw)
+unless they are specified, and the final mode is modified by the umask
+per standard *nix semantics.  If no mode is specified, it is set to
+the default and not modified.  Note that the perm option below is
+probably a better way to set this.
.TP
.B gmode=[0-7|[rwx]*]
Set the group file mode for the pty slave, see umode for details.
.TP
.B omode=[0-7|[rwx]*]
Set the other file mode for the pty slave, see umode for details.
.TP
.B perm=[0-7][0-7][0-7]
Set the full mode for the pty per standard *nix semantics, modified
by umask as the above mode operations are.
.TP
.B owner=<name>
Set the owner of the slave pty to the given user.
.TP
.B group=<name>
Set the group of the slave pty to the given group.
.SS "Remote Address String"
The remote address string the program and arguments run on the pty.
The argiuments will be a set of quoted strings with a space between each
string, one for each argument, with '"' around each argument, each '"'
in the string converted to '\\"' and each '\\' in the string
converted to '\\\\'.
.SS "Remote Address"
The address is a pointer to an integer, the ptym file descriptor is
returned.  addrlen must be set to sizeof(int) when passed in.
.SS "Local Address String"
This returns the device of the pty, generally /dev/pts/<n>.  This is
useful for pty gensios with no program, it allows you to get the value
for the other end to connect to.  Note that if you close and re-open a
pty gensio, you may be a different local address string.
.SH "msgdelim"
accepter =
.B msgdelim[(options)]
.br
connecting =
.B msgdelim[(options)]

A message delimiter converts an unreliable stream (like a serial port)
into an unreliable packet interface.  This is primarily to allow a
reliable packet interface like relpkt to run on top of a serial port.
It does not support streaming of data, so it's not very useful by itself.

Messages are delimited with a start of message and end of message and
have a CRC16 on the end of them.

This is primarily for use on serial ports and other streams that can
munge up the data.  Use the mux gensio for TCP and such.

THe default buffer size is 128 bytes, which is ok for a reasonably
fast serial port interface.
.SS Options
In addition to readbuf, the msgdelim gensio takes the following options:
.TP
.B writebuf=<n>
set the size of the write buffer.
.TP
.B crc[=on|off]
Enable/disable the CRC at the end of the packet.  Useful if you are
running over a reliable protocol, and especially for testing relpkt so
you can fuzz it and bypass the crc errors.
.SH "relpkt"
accepter =
.B relpkt[(options)]
.br
connecting =
.B relpkt[(options)]

Converts an unreliable packet interface (currently only UDP and
msgdelim) into a reliable packet interface.  This lets you have a
reliable connection over a serial port (with msgdelim) or UDP.

Note that UDP is not recommended, it doesn't handle flow control and
such in a friendly manner.

relpkt is unusual in dealing with clients and servers.  The protocol
is symmetric, for the most part, you can start two clients and they
will connect to each other, if they are started relatively close in
time to avoid one timing out.  A relpkt server will simply wait
forever for an incoming connection on an open.

relpkt does not support readbuf.  It supports the following:
.TP
.B max_pktsize=<n>
Sets the maximum size of a packet.  This may be reduced by the remote
end, but will never be exceeded.  This must be at least 5 bytes
shorter than the maximum packet size of the interface below it.  This
defaults to 123 (msgdelim max packet size - 5).  If run over UDP, this
should probably be increased for performance.
.TP
.B max_packets=<n>
Sets the maximum number of outstanding packets.  This may be reduced
by the remote end, but will never be exceeded.
.TP
.B mode=client|server
By default a relpkt is a server on an accepter and a client on a
connecter.  See the discussion above on clients and servers.
.SH "trace"
accepter =
.B trace[(options)]
.br
connecting =
.B trace[(options)]

A transparent gensio that allows the data passing through it to be
sent to a file.  Useful for debugging.

Note that the trace gensio only prints data that is accepted by the
other end.  So, for instance, if the trace gensio receives 100 bytes
of read data, it will deliver it immediately to the gensio above it.
If that only accepts 40 bytes, trace will only print 40 bytes and will
only accept 40 bytes from the gensio below it.  Same for sent data.
.SS Options
trace does not support readbuf.  It supports the following options:
.TP
.B dir=none|read|write|both
Sets what data is traced.  "none" means no data, "read" means data
flowing up the gensio stack (read by the parent), write means data
flowing down the gensio stack (written by the parent), and "both"
traces reads and writes.
.TP
.B raw[=yes|no]
If set, data will be written as raw bytes.  If not set, data will be
written in human-readable form.
.TP
.B file=<filename>
The filename to write to.  If not supplied, tracing is disabled.  Note
that unless
.B delold
is specified, the file is opened append, so it will add new trace data
onto the end of an existing file.
.TP
.B stderr[=yes|no]
Send the output to standard error.  Overrides file.
.TP
.B stdout[=yes|no]
Send the output to standard output.  Overrides file and stderr.
.TP
.B delold[=yes|no]
Delete the old data in the file instead of appending to the file.
.SH "perf"
accepter =
.B perf[(options)]
.br
connecting =
.B perf[(options)]

A gensio for measuring throughput for a connection.  This allows the
performance of a connection to be measured.  It does not pass any data
through.  Instead, it writes raw data to the lower layer (if
.B write_len
is set) and reads data from the lower layer, counting the bytes and
measuring the time.

To the upper layer, perf prints out statistics about the data
transfer.  It prints out the number of bytes written and read each
second, and at the end it prints a total.

If
.B write_len
and/or
.B expect_len
is non-zero, then the filter will return GE_REMCLOSE when it runs out
of write data and has received all expected data.  If both are zero,
the connection will not be closed by the gensio.
.SS Options
perf does not support readbuf.  It supports the following options:
.TP
.B writebuf=<n>
Sets the size of the buffer to write to the child gensio.  Each write
will be this size.  This defaults to zero.
.TP
.B write_len=<n>
The number of bytes to write.
.TP
.B expect_len=<n>
The number of bytes to expect from the other end.
.SH "conacc"
accepter =
.B conacc,<gensio string>

.B conacc
is a gensio accepter that takes a gensio as a child.  When the
accepter is started, it opens the child gensio.  When the child gensio
finishes the open, it reports a new child for the accepter.  The
reported gensio can be used normally.

When the gensio is closed, when the close completes the accepter will
attempt to re-open the gensio.  This means that if the gensio has some
sort of random address (like a pty or a tcp address with the port set
to zero) you can get a different address for the re-opened gensio.  So
you must refetch the local address or local port in this case.
.SH "mdns"
connecting =
.B mdns[(options)][,<name>]

This gensio uses mDNS to find a service and then attempts to connect
to it.  This can be convenient, it finds the connection type andport
for you, automatically adds telnet and rfc2217 if it's available.
The mDNS name can be set as an option or as the string after the comma
show above.
.SS Options
The readbuf option is accepted, but if it is not specified the default
value for readbuf will be taken for the sub-gensio is taken.  In
addition to readbuf, the mdns gensio takes the following options:
.TP
.B nostack[=true|false]
By default the mdns gensio will attempt to use the "gensiostack" text
string from the mDNS service.  If you don't want it to do this,
setting this option will turn it off.
.TP
.B name=<mdnsstr>
Set the mdns name to search for.  You generally want to set this,
otherwise you will get the first thing that comes up from the search.
.TP
.B type=<mdnsstr>
Set the mdns type to search for.
.TP
.B domain=<mdnsstr>
Set the mdns domain to search for.  You generally don't use this.
.TP
.B host=<mdnsstr>
Set the mdns host to search for.  You generally don't use this.
.TP
.B interface=<num>
Set the network interface number to search on.  The default is -1,
which means all interfaces.
.TP
.B nettype=unspec|ipv4|ipv6
The network type to search for.  unspec means any type, otherwise you
can choose to limit it to ipv4 and ipv6.
.TP
.B nodelay[=true|false]
Sets nodelay on the socket.  This will be ignored for udp.  Note that
the default value for mdns is ignored, if you don't set it here it will
take the default value for the sub-gensio that gets chosen.
.B laddr=<addr>
An address specification to bind to on the local socket to set the
local address.
.TP
.SH "Forking and gensios"
Unlike normal file descriptors, when you fork with a gensio, you now
have two unassociated copies of the gensios.  So if you do operations
on one, it might mess up the other.  Even a close might cause issues,
if you close an SSL connection, it sends data to the other end to
close the connection, even if the other fork doesn't want that.

To avoid issues with this, you should generally first make sure that
no thread is in a wait, service call, or any type of thing that would
examine file descriptors or timers and act on them.  This is
.B very
important, and you must do it
.B before
you fork.

Then after you fork, you should call:
.IP
gensio_disable(io)
.PP
on all the gensios that fork is not using, then free the gensios.
Don't use close, use free.  Then you should call:
.IP
gensio_acc_disable(acc)
.PP
on every gensio accepter that fork is not using, then free them.  If a
connection is in progress and has not been reported to the user, it
will be disabled then closed.

You cannot share a gensio between two different processes.  If a
gensio is used in one fork, it must be disabled and closed in the
other fork.

Another issue with forking on Linux is epoll.  An epoll fd is not
duplicated on a fork, both forks get the same epoll fd.  If you close
the epoll fd in one for, it will close it for the other.  To avoid
this issue, the os handler has a handle_fork() function that you must
call after a fork in the new fork (not the old one).  It will handle
any cleanup required after the fork.  Other systems may require other
cleanups after a fork, so you should always call this after a fork.
.SH "SEE ALSO"
gensiot(1), sctp(7), udp(7), tcp(7), unix(7)
.SH "KNOWN PROBLEMS"
None.
.SH AUTHOR
.PP
Corey Minyard <minyard@acm.org>
